1. Field of the Invention
The present invention relates to a method of automatically tracking and photographing celestial objects and a celestial-object auto-tracking photographing apparatus, each of which enables the capture of a freeze-frame picture of a celestial object(s) in long exposure astrophotography.
2. Description of the Related Art
If long exposure astrophotography is carried out with a fixed photographing apparatus, added light of stars during a long exposure form straight or curved light trails in the captured image since celestial objects move relative to the photographing apparatus due to the earth's rotation (diurnal motion).
To carry out a long exposure in order to photograph a celestial object so that the celestial object appears to be still (stationary; a light spot) relative to a photosensitive film or an image sensor (image pickup device) of a photographing apparatus, it is generally the case that a photographing operation (exposure operation) is performed while moving the photographing apparatus relative to the celestial object with use of an equatorial equipped with an auto tracking system. However, an equatorial equipped with an auto tracking system is generally expensive, heavy and difficult to handle; in addition, it is essential to carry out a polar alignment procedure in which the polar axis of the equatorial is brought into alignment with the north celestial pole.
In recent years, celestial-object auto-tracking photography has been proposed in which a photographing apparatus performs a photographing operation (exposure operation) while moving the imaging area of the imaging surface of an image sensor so that the celestial object(s) formed on the imaging surface of the image sensor via a photographing optical system of the photographing apparatus is fixed with respect to the imaging area of the image surface during a long exposure (Japanese Unexamined Patent Publication Nos. 2008-289052 and 2010-122672).
In the above-mentioned Japanese Unexamined Patent Publication No. 2008-289052, an image sensor (imaging area) is moved via a structure capable of rotating the entire movable stage that is capable of moving the image sensor in the X-direction and the Y-direction within a predetermined range of movement.
In Japanese Unexamined Patent Publication No. 2010-122672, an image sensor (imaging area) is moved via a structure capable of linearly moving the image sensor (imaging area) in directions orthogonal to an optical axis of a photographing optical system and rotating the image sensor (imaging area) about an axis parallel to the optical axis of the photographing optical system within a predetermined range of movement.
However, a high-performance CPU is required in order to compute, at a practical speed, information (e.g., information on motion of an image of a celestial object(s) on the imaging area and celestial object tracking information) for moving the imaging area of the imaging surface of an image sensor to thereby control an image of a celestial object(s) so as to become (appear) stationary with respect to the imaging area.
Specifically, to move the imaging area with linear movements (translations) of the imaging area in the X-direction and the Y-direction and rotational movements of the imaging area about an axis parallel to an optical axis of a photographing optical system being combined in such a way as disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2010-122672, an exposure time needs to be set within a range in which the imaging area does not move beyond the range of movement thereof in consideration of the range of the aforementioned linear movements and the range of the aforementioned rotational movements. This is because, if the imaging area moves beyond the range of movement thereof, tracking photography is no longer possible, which makes the CPU wastefully compute tracking data even on a range beyond an operable exposure time, which results in a further increase in the load on the CPU. In an embodiment utilizing an anti-shake drive unit for a photographing apparatus such as shown in the above-mentioned Japanese Unexamined Patent Publication No. 2010-122672, the range of movement of the imaging area is defined by mechanical limits between the anti-shake drive unit and an associated camera body.
However, the period of time from the commencement of an exposure to the moment the imaging area moves beyond the range of movement thereof varies depending on various factors such as the position of the imaging area at the commencement of the exposure, information on motion (moving direction, moving speed, rotational angle and rotational speed) of a celestial object image on the imaging area, the target celestial object(s), settings on the photographing apparatus, photographing date and time, and photographing location. Accordingly, it is difficult to set an exposure time within a range in which the imaging area does not move beyond the range of movement thereof at all times while such various factors momentarily change. In addition, to obtain a bright and sharp image, it is desirable to set as long exposure time as possible within a range in which the imaging area does not move beyond the range of movement thereof.